The invention concerns a method for measuring the spectrum of a measurement signal in several neighbouring channels adjacent to a working channel and a corresponding measurement device.
The standard ETSI Specs GSM 11.21 defines the maximum signal level of incidental emissions in the neighbouring channels adjacent to a working channel. To check whether mobile phone stations or base stations meet this standard, the measuring method according to the invention and the measuring device according to the invention are used. The measuring method and the device according to the invention are however also suitable in principle for measuring incidental emissions in neighbouring channels of signals, in particular mobile phone signals, to other standards in particular the EDGE standard or the UMTS standard.
The specification to ETSI Specs 11.21 requires a check of both the spectrum due to modulation and the spectrum due to switching. The GSM signal is divided as is known into several frames with a frame duration of approximately 4.7 ms. The said specification requires, for measurement of the spectrum due to modulation, the measurement of 21 channels (the working channel and neighbouring channels above and below the working channel) for at least 200 frames which are then averaged. If the measurements for each channel are performed separately, in total 21xc3x97200 measurements must be performed with a duration of at least 4.7 ms, so that the theoretical minimum measurement period is around 20 seconds. With automatic measurement of mobile stations or base stations for example at the end of production, this measurement period is relatively long, in particular as also further measurements of other specifications must be performed. This measurement duration can therefore hinder the production process.
In addition measurement of the spectrum due to switching must be performed. The said specification here requires measurement of 9 channels (the working channel and neighbouring channels above and below the working channel) for at least 20 frames.
The invention is therefore based on the task of producing a method for measuring the spectrum of a measurement signal in several neighbouring channels adjacent to a working channel and a corresponding measurement device in which the total measurement duration is substantially reduced.
The task is solved in relation to the method by the features of claim 1 and in relation to the measurement device by the features of claim 7. The sub-claims concern advantageous refinements of the invention.
The invention is firstly based on the knowledge that the measurement time can be significantly shortened if the individual measurement channels are measured not in series but in parallel. For this it is necessary to detect the broadband measurement signal and not restrict it before the analog/digital converter to the measurement bandwidth of the channel concerned (e.g. 30 kHz for the neighbouring channel and 300 kHz for the working channel) but supply the broadband measurement signal to the analog/digital converter. It has however been found that parallel detection of the measurement signal is not possible without a further measure according to the invention, as the signal strength in the neighbouring channels, in particular in the neighbouring channels relatively remote from the working channel, is approximately 65 dB lower than in the working channel. As the analog/digital converter only has a limited resolution and also due to the higher signal strength at the working channel must not be over-modulated, the problem arises as how these high dynamic requirements can be fulfilled. According to the invention therefore it is proposed to arrange a selective filter before the analog/digital converter which attenuates the measurement signal in the working channel substantially more than in the neighbouring channels. As a result an over-modulation of the analog/digital converter in the working channel is avoided and the measurement signal reaches the analog/digital converter in the neighbouring channels with a higher signal level. As a result the signal can be quantized relatively well even in the neighbouring channels. After the analog/digital converter is connected an equaliser, the frequency response of which is reciprocal to the frequency response of the selective filter. The influence of the selective filter on the measurement signal is thus compensated before analysis.
Advantageously the selective filter attenuates the measurement signal in the neighbouring channels increasingly weakly as the distance from the working signal increases, so that in each case the signal of the nearest neighbouring channels is attenuated even more than the signal in the more remote neighbouring channels. As a result the modulation of the analog/digital converter can be further improved.
In measuring the spectrum of the working channel it is advantageous to switch off the selective filter. As the signal of the working signal is in any case the dominant signal, in measurement of the working channel the selective filter offers no advantages but has the disadvantage that because of the lack of constancy of the frequency response within the 300 kHz wide measurement bandwidth in the working channel, i.e. at the summit of the filter curve, the measurement signal is falsified.
Connected before the analog/digital converter is preferably a variable amplifier or a variable attenuator, the amplification factor or attenuation factor of which is set so that the modulation range of the analog/digital converter is utilised at least almost completely without over-modulation.
The bandpass filter which reduces the bandwidth to the measurement bandwidth, i.e. for example to 30 kHz for the neighbouring channels and 300 kHz for the working channel, is preferably before the analog/digital converter. The equaliser and the bandpass filter are preferably housed in a digital signal processor which can also contain further signal processing elements.